Weird ribbing effect when applying Mirror and Subdivide modifiers on an object in 2.92
If you recreate the steps of the tutorial, the result looks exactly like in the tutorial. What your friend accidentally did is doubling the center edge by hitting E for Extrude. Maybe she realised her error and pressed Esc or RMB to exit the Extrude, but unlike moving, rotating or scaling for example, the extra vertices remain created - aborting the function only keeps them in place, so they are directly lying on top of the old ones. To get rid of them, use A in Edit Mode to select all vertices, then M &gt; Merge &gt; By Distance.
The horizontal edge looks like there is an extra edge loop created above or below the original center edge. But they are doing nothing like that in the tutorial.